The present invention relates to a transistor circuit for amplification or oscillation, and more particularly to a biassing circuit for the supply of a biassing voltage to the base of a transistor circuit.
A well known biassing circuit of this kind comprises a pair of resistors for dividing a power source voltage to supply the divided voltage to the base of the transistor circuit, as disclosed in the U.S. Pat. No. 3,974,452 issued on Aug. 10, 1976 (with particular reference to Oscillator 18 in FIG. 2b thereof) or in Motorola's Service Manual published in 1972 (more particularly pp. 8-10). The voltage dividing resistors requires high resistances to reduce the power consumption, with the consequence that the D.C. impedance of the biassing circuit is comparatively high and the voltage V.sub.BE between the base and the emitter of the transistor circuit declines with a rise in the input signal level, in the base.
Therefore, the biassing point of the transistor shifts from a linear region to a non-linear region, thereby varying the impedance between the base and the emitter. This results in a deterioration in the impedance matching characteristic of the transistor circuit and eventually in an abnormal oscillation or the like. This phenomenon can be avoided by reducing the resistances of the voltage dividing resistors however, the power consumption of the voltage divider would then increase.
Further, the variation in the collector current of the transistor circuit, owing to temperature change, can also be suppressed by reducing the resistances of the voltage dividing resistors, but here again there would be the disadvantage of a greater power consumption.